Blood-feeding patterns of Culiseta melanura and other potential vectors of EEE were determined in both swamp foci and nearby horse farms. Data suggest that the enzootic vector Cs. melanura is not a major vector of virus to horses and man during epidemics due to its strict avian feeding pattern. Culiseta melanura has 3 or 4 generations per season (ca. 1 month apart) according to parity and density data and it has both high nectar feeding and blood-feeding rates. Blood feeding occurs primarily during the evening post-twilight period and females fly actively at all elevations, even above the tree canopy. Both behaviors are consistent with a close association with avian hosts. Mosquitoes are attracted to and readily attack very young rodents but even blind and hairless newborns exhibit marked defensive behavior when mosquitoes land. Significant, however, is the fact that rodents infected with Plasmodium become much more receptive to attack, thereby increasing the probability of successful feeding and pathogen transfer. The black fly Simulium decorum has now been maintained in continuous colony in our laboratory of 15 generations and 18 months. At least 20 per cent of colony S. decorum females injected with microfilaria of Onchocerca gutturosa supported development to the infective 3rd larval stage. Nulliparous but autogenous flies do not develop more eggs if induced to blood feed or when given a blood enema. Studies on adult back fly natural resting habits are continuing with emphasis on visual orientation.